Security device with multiple authentication features

ABSTRACT

A security device comprising the following components:
     a) 30-80 wt % mono-functional LCP&#39;s,   b) 0-50 wt % higher functional LCP&#39;s, preferably below 50 wt %   c) 0-30 wt % liquid crystalline inert monomers, preferably below 20 wt %   d) 0-50 wt % non-liquid crystalline (mono- or higher) functionalized monomers, preferably below 30 wt %   e) 0-30 wt % non-liquid crystalline inert monomers, preferably below 20 wt %   f) 0.01-10 wt % initiators, preferably below 2 wt %   g) 0.-10 wt % inhibitors, preferably below 2 wt %   h) 0.01-50 wt % additives, preferably below 20%, more preferably below 10 wt %,
 
with the provisions that the total amount of the components is 100 wt %, characterized in that the additives h) comprise magnetic additives, such as paramagnetic, super-paramagnetic, diamagnetic or ferri-magnetic particles

The present invention pertains to a security device in particular asauthentication feature for prevention of counterfeiting of documentsand/or products.

In order to prevent counterfeiting, there is a continuing need to securevaluable documents and/or products. Adding authentication features,which are very difficult to forge but preferably easy to inspect, tothese products helps against counterfeiting. There are manyauthentication features known in the art. Many are based on well-knownand thus non-distinct optical effects. Examples of opticalauthentication features are holograms, based on diffracting structures,watermarks, based on thickness differences in semi-opaque substrates andfluorescent markings. Furthermore, gaining access to the materials andtechnology to produce these or highly similar effects is oftenstraightforward. The strength of authentication features produced withPolymerizable Liquid Crystals (LCP's) are the many possible opticaleffects, which are much less known and very difficult to imitate

Polymerizable Liquid Crystals (LCP's) are a class of materials whichexhibit one or more liquid crystalline phases, such as a nematic,smectic or chiral nematic (also called cholesteric) phase, within acertain temperature range. Furthermore, LCP's can be polymerized due toreactive groups which are part of the molecule. Before polymerization,LCP's are monomers, but also after polymerization the resulting polymersare commonly referred to as LCP's. In the text, where LCP's arementioned the monomer form is referred to; the polymer form is referredto as LCP polymer. Moreover, the skilled person is able to differentiatebetween the polymeric and monomeric LCP's in the context of thespecification and by using his common knowledge. Polymerization of LCP'scan be induced spontaneously at elevated temperatures or aided by meansof suitable initiators, such as for instance photo-initiators or thermalinitiators. Common examples of reactive groups are acrylates,methacrylates, epoxies, oxethanes, vinyl-ethers, styrenes andthiol-enes. Here, monomers which by means of reactive end groups havethe ability to form links with two other molecules are calledmono-functional, since two links are the minimum number required to forma polymer. Monomers with the ability to form links with more than twoother molecules are called higher functional.

It is possible to obtain security devices by printing flakes of LCPpolymers which are formulated into an ink, but these flakes areexpensive and difficult to produce as well as apply. Such flakes are forinstance disclosed in EP0968255. The production process of flakesconsists of a number of time-consuming steps before the flakes arepolymerized and broken into pieces of suitable size. Afterwards, theseflakes are mixed into a transparent polymerizable binder. Due to theaverage size of the flakes in the binder as well as the variability ofthe flake geometry, the print resolution is limited. Furthermore, flakesare not suited for inkjet printing as the flakes clog the conduits andnozzles in the inkjet head. Furthermore, the optical effects are not asuniform nor as clear and striking as a single layer of LCP.

Another method to create security devices containing LCP-basedauthentication features is by inkjet printing of LCP's to form featureswith distinct optical properties. Furthermore, inkjet printing allowsthat each print is made unique, which is especially useful for instancewhen a need exists to track and trace each individual document orproduct or to include specific information such as biometricinformation. The possibility of combining LCP-based optical effects andinkjet printing is only mentioned in very few documents, which are allnon-specific with regards to the actual requirements.

Creating security devices by printing LCP's with solvents has beendescribed e.g. in EP1381520. Solvents are materials which cause theLCP's to dissolve in them and together form a solution. Furthermore,such solvents are usually meant to evaporate after processing but beforepolymerization, so the solvent is not contained in any significantamount in the final product. Examples of commonly employed solvents forLCP's are xylene, toluene and acetone. The problem with ink jetting ofLCP's is that commercial inkjet printing equipment is unable toreproducibly print LCP's if these are dissolved in solvents. Ink jettingsolvent based mixtures leads to a drying effect which is detrimental tothe homogeneity of the prints, the so-called coffee stain effect.Furthermore the use of solvents can affect the substrates on which theLCP's are printed and also it can dissolve any previously printed LCPstructure on the substrate before polymerization. When printingstructures containing different materials stemming from differentreservoirs, the use of solvent based mixtures gives rise to significantmixing of the materials on the substrate, leading to effects such as‘colour bleeding’ or highly blurred images.

Solvents can also chemically attack parts of the print head or heads andconduits in the printing system, in particular polymer-based parts.These solvents also often evaporate at or near the nozzle, leading toclogging. Furthermore, such solvents are often harmful to the equipmentbecause of corrugation as well as to people and to the environment dueto toxicity and therefore require extensive equipment for air filtrationnear the printing equipment. Furthermore, heating of the printedmaterials to enhance evaporation and thus production speed is highlydesirable, requiring heating installations within the productionequipment which increase complexity and cost and also can be detrimentalto the surrounding equipment as well as the substrates on which isprinted.

Creating of security devices by printing of LCP's without solvents isalso known, e.g. from EP1491332. However, this document does notdescribe or teach specific materials or steps to enable the inkjetprinting of LCP's or potential problems related to it. In particular,mixtures containing LCP's but without solvents are hardly manageablewith the currently available production-ready equipment. Only withhighly dedicated inkjet printing equipment is it possible to printLCP's, and only at or above 140C, limiting the scope of security devicesto be produced At such temperatures however, LCP's start to polymerizespontaneously through thermal excitation of the reactive groups or theinitiators in the mixture.

To be able to print a mixture with inkjet printing it is needed that themixture which is printed is at least chemically stable for the time itremains in the inkjet head or reservoir at the temperature at which themixture is printed, which is in the order of minutes to weeks dependingon the printing speed an the printer usage.

It is the object of the present invention to overcome the limitations ofthe prior art by providing a security device, which can be reproduciblyprinted with state-of-the art equipment, with the inclusion of variousadditives.

It is one object of the invention to provide a security device that caneasily and reproducibly be made by common printing technologies and atthe same time does not show the restriction of the security devices inthe prior art.

It is another object of the inventions is to show provide for new LCPbased security features by adding one or more of a range of specificadditives to the LCP materials. These materials can add otherauthenticating effects to the resulting authentication feature, thuscreating a new authenticating feature.

It is another object of the invention to provide a security device thatcan be manufactured via printing techniques, which allows for thecreation of arbitrary structures which can contain information in theform of e.g. barcodes, images, text, numerical codes.

Surprisingly, it has been found that the object of the present inventioncan be achieved with a security device comprising the followingcomponents:

-   -   a) 30-80 wt % mono-functional LCP's    -   b) 0-50 wt % higher-functional LCP's    -   c) 0-30 wt % liquid crystalline inert monomers, preferably below        20 wt %    -   d) 0-50 wt % non-liquid crystalline (mono- or higher)        functionalized monomers, preferably below 30 wt %    -   e) 0-30 wt % non-liquid crystalline inert monomers, preferably        below 20 wt %    -   f) 0.01-10 wt % initiators, preferably below 2 wt %    -   g) 0-10 wt % inhibitors, preferably below 2 wt %    -   h) 0.01-50 wt % additives, preferably below 20%, more preferably        below 10 wt %,        with the provisions that the total amount of the components is        100 wt %, wherein the additives h) comprise magnetic additives,        such as paramagnetic, super-paramagnetic, diamagnetic or        ferri-magnetic particles.

In the following, some non-limitative examples are given of suitablecomponents.

An example of category a) is

where x is between 1 and 10.

Suitable examples from category d are

and also

as well as

Acrylates have specific advantages in that they have fast curing timesas well as a wide choice in suitable initiators. However LCP's withdifferent reactive groups are also known from literature, and could alsobe applied with the same method. Epoxies for instance have the specificadvantages that they provide for good adhesion to many printed surfaces,exhibit relatively small amounts of polymerization shrinkage and areless affected by oxygen inhibition during polymerization. Oxethanes haveproperties comparable to epoxies, but typically exhibit lesspolymerization shrinkage. Thiol-ene based polymerization (including boththiol and -ene groups in the molecule structure) is also possible.Vinyl-ethers are also not very sensitive to oxygen inhibition andpolymerize very rapidly, but only at relatively high temperatures.

A suitable example of category e) is ethylene glycol.

Suitable examples of category f) are for instance sold by Ciba-Geigy(Switzerland), under the trade names Irgacure and Darocure, and ingeneral polymerization initiators are known in the art.

It is further preferred if components or a mixture are applied to asubstrate by means of printing, such as but not limited to inkjetprinting, flexography, offset printing, screen printing, micro-contactprinting, micro-transfer printing, gravure printing, rotogravureprinting, reel-to-reel printing.

Examples of magnetic additives are paramagnetic, super-paramagnetic,diamagnetic or ferri-magnetic particles. Such particles are typically 5to 500 nm in size. The magnetic properties can be probed by e.g.magnetoresistive sensors. The addition of such particles enables thecreation of structures that can be moved mechanically by means ofmagnetic fields. Such movement can give rise to e.g. altered optical,mechanical, electrical or magnetic properties of the printed structure,which can be used to authenticate the feature.

It is particularly beneficial that the printed structures are (in part)made from LCP's, since the anisotropic properties of the aligned LCPpolymer matrix can enhance the magnetic and mechanical propertiesdesired to fully exploit the magnetic properties of the print.Especially if the magnetic material is magnetically anisotropic and alsoaligning with regards to the LCP polymer matrix. The anisotropicmagnetic properties will then be macroscopically available.

A preferred embodiment comprising a magnetuc additive is shown by thefollowing components:

-   -   a) 54.5 wt % mono-functional LCP acrylate

-   -   b) 22 wt % di-functional LCP acrylate

-   -   c) 19 wt % non-reactive LC monomer K15

-   -   f) 1.0 wt % photo-initiator

-   -   g) 0.5 wt % inhibitor hydroquinone

-   -   h) 3 wt % of magnetic microspheres beads of mean diameter 0.9 μm        diameter containing between 20 and 60% magnetite in a        polystyrene/divinylbenzene matrix

Components a through h are added in a glass vial and diluted withparaxylene with a ratio of 1 (mixture):1.25 (paraxylene), and theresulting mixture is manually stirred for 5 minutes at 70° C. Theresulting mixture is then fluid. This mixture is printed in a 10 μm filmwith the doctor blading technique on a tri-acetylcellulose film, whichis rubbed with a velvet cloth to induce alignment. The solvent film isallowed to evaporate at 50° C. during 2 minutes and the resulting filmis then UV-cured in a nitrogen atmosphere.

The resulting feature is birefringent and contains magnetic particleswhich are evenly dispersed in the polymer matrix.

It is further preferred that the additives h) comprise additionally oronly conductive or semi-conductive additives, more preferably,conductive or semi-conductive additives that are selected from a groupcomprising nanometer or micrometer sized rods, flakes, spheres orotherwise suitably shaped conductive particles of metals, alloys orsemiconductor-based materials, and even more preferably conductive orsemi-conductive additives are selected from a group comprisingsemi-conductive conjugated polymers, such as polyphenylene vinylene ormixtures of polymers such as PEDOT-PSS (a mixture ofPoly(3,4-ethylenedioxythiophene) and sodium poly(styrenesulfonate)), andsemi-conductive liquid crystals, such as oligothiophenes, which arepreferably LCP's.

A particular benefit of adding (semi-) conductive or magnetic additivesto the prints is that the authentication is straightforward by means ofelectric and magnetic fields or currents, and the effects can bereversible enabling non-destructive authentication. Furthermore, aparticular benefit of inkjet printing such structures is that theseadditives can be printed in varying structures, thus enabling unique andidentifiable responses to electrical or magnetic fields.

Conductive additives enable the printing of electronic circuits. Suchcircuits can be used for instance to create optical effects which areswitchable by means of electrical signals. The conducting properties ofthe structure itself too can be used as an authentication feature. Thiscan be done particularly effectively if elements of electronic circuits,such as FET's, diodes or capacitors are created within the print, sincethese give rise to designable and clearly identifiable electronicresponses. It is possible that the conductive structures are used tomake switchable another, adjacent non-conductive printed structure,either of which is not necessarily but preferably applied by means ofinkjet printing. Such multi-layered prints are advantageously createdsequentially or concurrently, either in a single layer or in separatelayers, printed either on top of or next to each other or even onopposite sides of the substrates or on multiple substrates which areassembled together after printing. Furthermore, it is possible to createstructures which are conductive and contain electroluminescent orelectrochromic additives, which can be addressed (made to change theoptical appearance of the feature) by currents flowing through theprinted structure itself. Furthermore, by supplying charges of equal oropposite sign to two electrically isolated by adjacent parts of thestructure, capacitors can be formed. If such parts of the structure areable to move mechanically, such movement can give rise to e.g. alteredoptical, mechanical, electrical or magnetic properties of the printedstructure, which can be used to authenticate the feature.

It is particularly beneficial that the printed structures are (in part)made from LCP's, since the anisotropic properties of the aligned LCPpolymer matrix can enhance the electrical and mechanical propertiesdesired to fully exploit the conductive properties of the print.

It is further preferred that the security device according to theinvention comprises additionally or only additives h) in the form ofphotochromic pigments or dyes, thermochromic pigments or dyes,electrochromic pigments or dyes, ionochromic pigments or dyes,halochromic pigments or dyes, solvatochromic pigments or dyes,trobochromic pigments or dyes and piezochromic pigments or dyes,fluorescent of phosphorescent pigments or dyes.

It is further preferred that these optical additives show anisotropicoptical properties and that they align with the LCP material in thesecurity device. This gives the entire security device additionalanisotropic optical properties.

It is preferred for inventive security device if the components or amixture of the components are applied to a substrate by means ofprinting, such as but not limited to inkjet printing, flexography,offset printing, screen printing, micro-contact printing, micro-transferprinting, gravure printing, rotogravure printing, reel-to-reel printing.

In addition, it is preferred for the security device that during themanufacturing process manufacture the components are aligned by asubstrate layer comprising linearly photo-polymerizable polymers,preferably by aligned multiple types of alignment through thecombination of multiple aligning substrates.

Preferably these substrates contain further authentication features,such as holograms, retro-reflecting layers, interference stackreflectors, fluorescent layers, color-shifting layers or featuresprinted by means of flakes.

It is preferred for the security device if the components from e)through h) are selected such that they do not prohibit the alignment ofthe liquid crystals from components a) through d).

Preferred security devices contain components from a), b) and d) thatare functionalized to form a polymer structure, preferably due topossessing compatible types of functional groups, very preferable bypossessing the same type of functional group.

Moreover, the inventive security device preferably comprises componentsa) through h) that are selected such that phase-separation is suppressedat least only before polymerization but preferably also duringpolymerization.

By applying such components printing is easily possible below 120° C.,preferably below 100° C., very preferably 80° C., to achieve securitydevices with distinctive and reproducible optical effects. Inparticular, it is also possible to add solvents to mixtures of thesecomponents which makes the mixture applicable to various printingtechniques.

Combinations of different initiators can be used which has a number ofpotential advantages, of which a number are now described.

Different initiators will give different mechanical properties of themixture before and after polymerization. Using mono-functionalinitiators can allow for a low viscosity of the mixture. However,multifunctional initiators tend to be more viscous. The exact choice ofinitiators also influences e.g. the flexibility and toughness of thepolymerized structure. Furthermore, the choice of photo-initiator isdependent on the presence of other absorptive additives in the mixture,which will be described below at category h). It is possible to usemixtures of initiators which in these cases too allow for a goodpolymerization of both the top layer of the structure as well asthroughout the structure. An initiator that can overcome oxygeninhibition of the surface can be used to initiate polymerization at thesurface, whereas another initiators can be used to through-cure thestructure. It is also possible to choose initiators in such a way thatonly the top of the structure polymerizes, but the remaining partremains non-polymerized. The non-polymerized LCP is then still fluid,and can be manipulated by means of for instance electric or magneticfields, i.e. they remain switchable.

The list of other possible additives from category h) contains, but isnot limited to, the following examples.

One example of additives are molecules that can act as tracer molecules,such as e.g. DNA-molecules. These can be added in minute quantities,typically with amounts in the order of one part per million (ppm), sothat they are very difficult to trace when their exact properties arenot known beforehand.

Another example of additives are surfactants, which can enhancealignment of the liquid. These surfactants can either enhance thealignment of the liquid crystalline matrix at the top of the structure,at the bottom or in the bulk or in combinations of those locations.Furthermore, these surfactants can influence the potential phaseseparation of the mixture or influence the mechanical properties (e.g.viscosity, surface tension) of the mixture on the substrate or canperform a combination of these three functions.

Another example of additives are chiral additives, commonly referred toas chiral dopants, which are used to induce a chiral phase in the liquidcrystal structures. Such chiral structures reflect light from a specifictunable wavelength range and with a specific handedness, which caneither be commonly called left- or right-handed, and the handedness isdetermined by the handedness of the chirality of the structure. It isnoted that one or more of the LCP's from categories a) through c) canalso exhibit intrinsic chiral phases, and/or induce such a phase for allthe liquid crystal molecules. By making use of chiral LCP's, a chiralstructure can be obtained without the use of chiral additives. Manychiral additives are known to the person skilled in the art. Preferablythe chiral additives are also polymerizable, which has the advantagethat the temperature dependence of the reflected wavelength is usuallydecreased.

Other examples of additives are pigments and dyes. Pigments and dyes canbe added to give the mixture an intrinsic color by means of absorptionof part of the spectrum as well as luminescence in part of the spectrum.Such intrinsic color can enhance the optical effects of the printedstructure, for instance by enhancing contrast of (parts of) the printedstructure.

Pigments are particles which do not dissolve molecularly in the mixturewhereas dyes can be approximately molecularly dissolved. The choicebetween pigments and dyes is dependent on various factors. One importantfactor is the solubility of the dyes or the pigments, with or withoutthe aid of a dispersant, to create a stable ink. Solutions with dyes aregenerally easier to process than dispersions with pigment, but theoptical properties of pigments are usually more stable. Furthermore,certain optical additives are only available as pigments and not asdyes, such as di-electric stacks, whose optical effects are not based onmolecular effects, but on effects on a larger scale. Another importantfactor is the price of pigments, which is usually higher than that ofdyes.

Examples of absorbing pigments or dyes are for instance

-   -   Absorbing only, meaning that a specific part of the spectrum is        absorbed    -   Photochromic pigments or dyes, which by excitation with light of        a particular part of the spectrum reversibly change into another        chemical species having a different absorption spectrum from the        original chemical species. Non-reversible photochromic pigments        and dyes also exist for specific purposes    -   Thermochromic pigments or dyes, which exhibit a reversible        change in absorption spectrum through the application of heat        (i.e. at raised or lowered temperatures). Non-reversible        thermochromic pigments and dyes also exist for specific purposes    -   Electrochromic pigments or dyes, which exhibit a change in        absorption spectrum through the addition of electron charges    -   Ionochromic pigments or dyes, which exhibit a change in        absorption spectrum through the addition of ionic charges.    -   Halochromic pigments or dyes, which exhibit a change in        absorption spectrum through changes in pH.    -   Solvatochromic pigments or dyes which exhibit a change in        absorption spectrum through changes in the polarity of the        solvent which is in contact with them.    -   Tribochromic pigments or dyes, which exhibit a change in        absorption spectrum as a result of friction applied to them.    -   Piezochromic pigments or dyes, which exhibit a change in        absorption spectrum through changes in the pressure applied to        them.

Examples of luminescent pigments or dyes are for instance

-   -   Fluorescent pigments or dyes, which exhibit absorption of light        in a particular part of the spectrum and emission in another        part of the spectrum, typically at a lower wavelength, where the        absorption and emission of individual photons occur subsequently        but with delays of typically nano-seconds.    -   Phosphorescent pigments or dyes exhibit similar absorption and        emission as fluorescent dyes, but due to a different quantum        mechanical decay mechanism typically emit photons after        absorption with much larger delays of up to hours or days.    -   Chemoluminescent pigments or dyes, which exhibit emission of        photons as a result of chemical reactions of the pigments and        dyes. Such reactions are generally non-reversible.    -   Electroluminescent pigments or dyes, which exhibit emission of        photons as a result of radiative recombinations of electrons and        holes within the pigments or dyes. Such radiative recombination        can occur if an electric current is passed through the pigments        or dyes, or alternatively of they are subjected to strong        electric field capable of exciting electron-hole pairs which        subsequently recombine.    -   Triboluminescent pigments or dyes, which exhibit emission of        photons as a result of friction applied to them.    -   Piezoluminescent pigments or dyes, which exhibit emission of        photons as a result of pressure applied to them.    -   Radioluminescent pigments or dyes, which exhibit emission of        photons as a result of ionizing radiation, such as beta        particles, applied to them.

There are also pigments or dyes which combine multiple optical effectswithin a single additive, or which in fact an effect which is related tomultiple causes concurrently. Examples are

-   -   Thermochromic pigment capsules which change colour if heated        above a certain threshold temperature. At this temperature the        crystalline solvent in the capsule melts and effectively lowers        the pH. This in turn causes the halochromic compound present to        change its absorptive properties.

Photochromic fluorescent dyes are dyes which exhibit fluorescence onlyafter the molecule has absorbed photons from a part of the spectrumwhich it does not absorb in its subsequent fluorescent state. Thiseffect which is concurrently photochromic and fluorescent, i.e. due tothe first absorption not only the absorptive properties of the moleculechanges (photochromism) but also the molecule subsequently exhibitsfluorescence or a change in its fluorescent properties

A preferred embodiment comprising a phosphorescent additive is shown bythe following components:

a) 53.5 wt % mono-functional LCP acrylate

b) 21.5 wt % di-functional LCP acrylate

c) 18.5 wt % non-reactive LC monomer K15

f) 1.0 wt % photo-initiator

g) 0.5 wt % inhibitor hydroquinone

h) 5 wt % of phosphorescent pigment PPSB-03, from RiskReactor,California, USA,

Components a through h are added in a glass vial and diluted withparaxylene with a ratio of 1 (mixture):1.25 (paraxylene), and theresulting mixture is magnetically stirred for 5 minutes at 70° C. Theresulting mixture is then fluid. This mixture is printed in a 40 μm filmwith the doctor Blade technique on a tri-acetylcellulose film, which isrubbed with a velvet cloth to induce alignment. The solvent film isallowed to evaporate at 50° C. during 2 minutes and the resulting filmis then UV-cured in a nitrogen atmosphere.

The resulting feature is birefringent and contains phosphorescentparticles which are evenly dispersed in the polymer matrix.

When the resulting feature is viewed between crossed polarizers, thebirefringence is clearly visible. When the alignment axis of the featureis parallel to either polarizer, the feature is dark between thepolarizers. When the alignment axis of the feature is at 45 degrees toboth polarizers, the feature is bright between the crossed polarizers.When the feature is placed under a UV light, the phosphorescentparticles lights up bright orange. When the UV light is turned off, thebright orange glow remains for about half a minute.

A preferred embodiment comprising a photochromic additive is shown bythe following components:

a) 53.5 wt % mono-functional LCP acrylate

b) 21.5 wt % di-functional LCP acrylate

c) 18.5 wt % non-reactive LC monomer K15

f) 1.0 wt % photo-initiator

g) 0.5 wt % inhibitor hydroquinone

h) 5 wt % of photochromic blue pigment (IT9 014) from MUTR, Herts, UK

Components a through g are added in a glass vial and diluted withparaxylene with a ratio of 1 (mixture):1.25 (paraxylene), and theresulting mixture is magnetically stirred for 5 minutes at 70° C. Theresulting mixture is then fluid. Component h is then added, dissolved inethanol with a ratio of 1 (component h):10 (ethanol). The mixture isthen vigorously stirred magnetically for 30 seconds. This mixture isprinted in a 40 μm film with the doctor Blade technique on a tri-acetylcellulose film, which is rubbed with a velvet cloth to induce alignment.The solvent film is allowed to evaporate at 50° C. during 2 minutes andthe resulting film is then UV-cured in a nitrogen atmosphere.

The resulting feature has the pigment dispersed throughout the polymermatrix. The resulting feature is birefringent, but due to the dispersedpigment it is also scattering. The birefringence is visually inspectedbetween crossed polarizers. When the alignment axis of the feature isparallel to either polarizer, the feature is dark but the scattering dueto the pigments gives rise to some transmission of light. When thealignment axis of the feature is at 45 degrees to both polarizers, thefeature is bright between the crossed polarizers. The resulting featurehas a yellow colour, when the resulting feature is placed in a UV-lightfor 30 seconds, the colour turns to blue. When the feature is no longerin the UV-light it retains the blue colour for a few minutes and thenreturns to yellow again.

A preferred embodiment comprising a thermochromic additive is shown bythe following components:

A preferred embodiment are the following components:

a) 53.5 wt % mono-functional LCP acrylate

b) 21.5 wt % di-functional LCP acrylate

c) 18.5 wt % non-reactive LC monomer K15

f) 1.0 wt % photo-initiator

g) 0.5 wt % inhibitor hydroquinone

h) 5 wt % of thermochromic pigment IT9 007 “magenta” from MUTR, Herts,UK

Components a through g are added in a glass vial and diluted withparaxylene with a ratio of 1 (mixture):1.25 (paraxylene), and theresulting mixture is magnetically stirred for 5 minutes at 70° C. Theresulting mixture is then fluid. Component h is then added, dissolved inethanol with a ratio of 1 (component h):10 (ethanol). The mixture isthen vigorously stirred magnetically for 30 seconds. This mixture isprinted in a 40 μm film with the doctor blading technique on atri-acetylcellulose film, which is rubbed with a velvet cloth to inducealignment. The solvent film is allowed to evaporate at 50° C. during 2minutes and the resulting film is then UV-cured in a nitrogenatmosphere.

The resulting feature has the pigment dispersed throughout the polymermatrix. The resulting feature is birefringent, but due to the dispersedpigment it is also scattering. The birefringence is visually inspectedbetween crossed polarizers. When the alignment axis of the feature isparallel to either polarizer, the feature is dark but the scattering dueto the pigments gives rise to some transmission of light. When thealignment axis of the feature is at 45 degrees to both polarizers, thefeature is bright between the crossed polarizers. When the resultingfeature is at room temperature it has a red colour, when the feature isheated to 50° C. the red colour disappears. When the feature is cooleddown to room temperature the red colour re-appears.

A preferred embodiment comprising a fluorescent additive and magneticparticles is shown by the following components:

-   -   a) 53.3 wt % mono-functional LCP acrylate

-   -   b) 21.6 wt % di-functional LCP acrylate

-   -   c) 18.6 wt % non-reactive LC monomer K15

-   -   f) 1.0 wt % photo-initiator

-   -   g) 0.5 wt % inhibitor hydroquinone

h1) 3 wt % of magnetic microspheres beads of mean diameter 0.9 μmdiameter containing between 20 and 60% magnetite in apolystyrene/divinylbenzene matrixh2) 2% DFSB-K44-65 fluorescent dye from RiskReactor, California, USA.

Components a through h2 are added in a glass vial and diluted withparaxylene with a ratio of 1 (mixture):1.25 (paraxylene), and theresulting mixture is manually stirred for 5 minutes at 70° C. Theresulting mixture is then fluid. This mixture is printed in a 20 μm filmwith the doctor blading technique on a tri-acetylcellulose film, whichis rubbed with a velvet cloth to induce alignment. The solvent film isallowed to evaporate at 50° C. during 2 minutes and the resulting filmis then UV-cured in a nitrogen atmosphere.

The resulting feature is birefringent and contains magnetic particleswhich are evenly dispersed in the polymer matrix. The birefringence isvisually inspected between crossed polarizers. When the alignment axisof the feature is parallel to either polarizer, the feature is dark.When the alignment axis of the feature is at 45 degrees to bothpolarizers, the feature is bright between the crossed polarizers. Undera microscope the dispersed magnetic particles are clearly visible. Whenplaced under a UV-lightsource it shows bright yellow fluorescence. Thisfluorescence is isotropic in emission and absorption.

Pigments and dyes can exhibit anisotropic optical properties, dependingon their molecular orientation. If anisotropic dye molecules align to asignificant degree within the LCP matrix, typically parallel orperpendicular to the LCP alignment, typically caused by a distinctanisotropic molecular shape, these molecules can exhibit theiranisotropic optical properties collectively, leading to distinctiveoptical effects, which remain after polymerization of the LCP matrix.This effect is commonly known as dichroism or pleochroism. Pigments canalso exhibit dichroic effects if the particles as such have anisotropicoptical properties. However, such properties are difficult to exploitsince for a collective effect all pigments have to be effectivelyaligned in the direction of their inherent anisotropy.

It is possible to create features which exhibit fluorescent dichroism inabsorption but not in emission or vice versa. This effect can beachieved for instance by using two fluorescent molecular species, one ofwhich absorbs and emits essentially non-dichroic and the otheressentially dichroic. By choosing both species in such a way that theabsorbed photon-energy is transferred to the other species, such effectscan be obtained. Also, fluorescent molecules can exhibit differentdegrees of dichroism in absorption and emission, but the effect withusing multiple suitably chosen species is in general more pronounced.

A preferred embodiment comprising an anisotropic fluorescent additive isshown by the following components:

-   -   a) 54.5 wt % mono-functional LCP acrylate

-   -   b) 22 wt % di-functional LCP acrylate

-   -   c) 19 wt % non-reactive LC monomer K15

-   -   f) 1.0 wt % photo-initiator

-   -   g) 0.5 wt % inhibitor hydroquinone

-   -   h) 3 wt % of an anisotropic fluorescent dye DFSB-K82 from        RiskReactor, California, USA.

Components a through h are added in a glass vial and diluted withparaxylene with a ratio of 1 (mixture):1.25 (paraxylene), and theresulting mixture is magnetically stirred for 15 minutes at 70° C. Theresulting mixture is then fluid and clear. This mixture is printed withprinted in a 10 μm film with a doctor blading technique on atri-acetylcellulose film, which is rubbed with a velvet cloth to inducealignment. The solvent film is allowed to evaporate at 50° C. during 2minutes and the resulting film is then UV-cured in a nitrogenatmosphere. The resulting film is birefringent. When placed under aUV-lightsource it shows bright yellow fluorescence. This fluorescence isanisotropic in emission: when the emission is inspected visually througha polarizing filter the emission is high for one polarization directionand low for the orthogonal polarization direction. The differencebetween the high and low emission is optically very distinctive, givingthe security feature a striking effect.

When using the same mixture described above, only with differentfluorescent dyes, we have also obtained layers that show:

-   -   isotropic fluorescence in both emission and absorption with dye        DFSB-K44-65 from RiskReactor, California, USA,    -   anisotropic fluorescence in both emission and absorption with        dye DFSB-K61.

The anisotropy of the emission is inspected optically by viewing theemission through a polarizing filter which is rotated, while theUV-source is not polarized. The anisotropy of the absorption isinspected optically: a UV-polarizing filter is placed in front of the UVsource and the sample is then rotated, while the emission is inspected.

UV-absorbing pigments and dyes or pigments can serve several specificpurposes. Such UV-protecting pigments and dyes can be present in theprinted mixture or applied over the printed structure after curing byanother printing step, preferably by means of flexography or offsetprinting. Also other application methods can be employed, such asbar-coating, doctor blading, spraying or by applying a UV-absorbingsubstrate on top of the printed substrate.

As these pigments or dyes absorb UV light, they can protect the printedlayer, or the substance underneath this layer, from harmful UV-radiationwhich can lead to degradation of the (mechanical) properties of thestructures, such as brittling. During the UV-curing of the printedlayer, UV-absorbers can also be used to prevent the deeper parts of thelayer of being polymerized, thus allowing for a non-polymerized layer toexist, whereas the top layer is solidified during polymerization. Also,such a non-polymerized layer is not created but there is formed agradient in the structure if specific components of the mixture diffusetowards or away from the higher polymerized regions duringpolymerization. Such gradients create new optical effects. For instance,a gradient in the amount of chiral dopant leads to structures afterpolymerization which exhibit a gradient in the chiral pitch, thusreflecting light over a greater wavelength range than a single pitchedstructure would. This effect is known as a broadband cholesteric mirror.

Again it has to be emphasized that the additives mentioned above caneither be present together or separately in the mixture. It goes withoutsaying, however, that the total amount of components forming thepolymerizable mixture is always 100 wt %.

As the anisotropic optical properties of the LCP's in the matrix beforeand after polymerization are dependent on their alignment, the preferredsubstrates on which is printed induce or do not interfere with thealignment of the LCP's so as to obtain the desired optical effect.Commonly used substrates are rubbed polyimide, as well as rubbedtri-acetyl-cellulose, polyethylene terephthalate, polyethylene orpolypropylene. Rubbing causes planar aligning properties for thesesubstrates.

Other substrates, also substrates causing other types of homogenousalignment, are known in the art as well. Common types of alignment aree.g. planar, homeotropic and tilted alignment.

LPP (Linearly Photopolymerizable Polymers) layers can also be used asalignment layers. LPP allows for the patterning of the alignment layerby means of polarized light and thus multi-domain patterning of thealignment layer. Furthermore it is possible to use self-assembledmono-layers (SAM's) as alignment layers, which can easily be applied ina pattern by e.g. printing. Combinations of for instance SAM's and LPPor tri-acetyl cellulose layers allow for an increased control over thealignment of the LCP's in the azimuthal and polar direction.

Next to the aligning properties of the surfaces, the choice insubstrates also determines the interactions between the mixture and thesubstrate. These interactions can be used to create additional (optical)effects. E.g. the use of hydrophobic of hydrophilic (chemically)patterned surfaces allows for print confinement and thus a higher printresolution and more striking optical effects. A geometrically patternedsurface can also be used to confine printed ink. Confinement can lead toprinted structures with more controlled geometries, leading to betterdefined properties which are beneficial for authentication purposes.Such chemical or geometrical patterning of the substrates can beachieved by means of printing, but also other techniques such as forinstance embossing, rubbing and lithography.

The optical properties of the employed substrates influence the overallproperties of the security feature. Such substrates can be combined,i.e. stacked on top of each other creating a multi-layered securityfeature, or a security feature created on a stack of substrates eachhaving particularly beneficial properties. Dependent on the preferredoptical effects, the substrates can be transparent, absorbing in anyrange of wavelengths, scattering or reflecting or can comprise patternsof these effects. The substrates can also have other optical properties.Examples are the ability to transmit only one polarization, as is thecase with polarization films which transmit only one linearpolarization, or the ability to reflect only one polarization, e.g.cholesteric films only reflect one handedness of light. Furthermore thesubstrates can change the polarization of transmitted or reflectedlight, as is the case with for instance retarder films and half waveplates.

The substrates can also contain other authentication features. Examplesare holograms, retro-reflecting layers, interference stack reflectors,fluorescent layers, color-shifting layers or features printed by meansof flakes. It is also possible to add layers containing otherauthentication features on top of the LCP polymer structures, via e.g.lamination.

It is preferred that the as-produced features are created such that theycan be applied as tamper evident labels to products or documents. Suchlabels have properties which render the intact removal of the labelsvery difficult. Such properties could be poor mechanical integrity, forinstance features which have low toughness, i.e. low resistance totearing. Furthermore, the features upon removal can leave behind cleartraces of its previous presence, for instance by means ofrupture-sensitive ink particles.

It is also preferred that the features can easily be applied to thedocuments and products. Such application can for instance be by means ofhot-embossing or by creating self-adhesive features.

Printing of LCP's is a very flexible technique, which allows for manynovel optical designs, for instance by using multiple inks which allowsfor the concurrent printing of inks with different optical properties.Of course it is also possible to print on top of or underneath layersprinted via different printing technique's, such as flexography, laserprinting, offset printing, screen printing, micro-contact printing,micro-transfer printing, intaglio printing, gravure printing,rotogravure printing, reel-to-reel printing, and thermal transferprinting.

Such printing could be done in series or parallel depending on theprinting equipment design. All these options enable a great host ofembodiments with specific uses for specific purposes. In the followingparagraph a few of the options are mentioned, although this is not anexhaustive list of the possibilities of printing LCP's.

The combination of normal black or colored inks with LCP inks, can allowfor the inclusion of overt features on top of normal printed informationby printing retarding structures on top of an image or text printed withregular ink on a reflection substrate. It can also allow for theenhancement or decrease of contrast by contrast by printing cholestericLCP mixtures on top of an image.

By using inks with identical apparent color but with differentreflective properties, such as left-hand and right-hand reflectingcholesteric LCP's, hidden information can be printed which can only berevealed using a polarization sensitive device or machine reader. Thecombination of color-shifting non-liquid crystal inks and color shiftingLCP ink creates the identical effect, as one ink has a uniformreflection and the other is polarization selective. The combination ofinks with various additives, but with the same optical properties alsoallows for hidden information to be printed.

Printing several layers on top of each other can enhance reflectivity orenhance the colors printed.

Self-authenticating structures can be printing by using e.g. structuredpolarizers and a LC structure on a reflective surface, which can befolded together to create additional effects. This can also recoverencrypted data.

As is already clear from the above descriptions, it is a particularadvantage of inkjet printing of LCP's that it is possible to createauthentication features with (multiple) overt, covert, forensic andbiometric levels of security, which will be explained below. Also, it ispossible to use one of multiple options or combinations of options ineach level by suitably choosing (combinations of additives such aspigments and dyes, substrates, printing procedures and designs of thelayers present in the authentication features.

Features which incorporate levels of security that are directly apparent(in particular visible) to anyone without the use of additional devices,like the color-shifting effect, which can be seen if a printed chiralnematic structure is tilted, are known as overt. Any overt securityfeature should be easily recognizable, but very difficult tocounterfeit. Other examples are multi-color prints of liquid crystalcolor-shifting inks.

Covert levels of security can only be made apparent using simpledevices. For instance, the birefringent nature of LCP's can be revealedusing simple devices such as polarizers if choosing suitable designs.Examples are nematic structures printed on a reflecting substrate, whichshows reflection or no reflection when viewed through an polarizer at 0or 45 degrees to the alignment axis. Another example is a chiral nematicprint, which reflects only one handedness of circularly polarized light.This can be discerned by a circular polarizer. Parts of the printedstructure which could be left non-polymerized could beelectromagnetically switched if having anisotropic dielectric constants,as is the case for many inert liquid crystals. This changes the opticalproperties of the structures. In use it is sometimes preferred that thecovert level is not visible as a security feature (i.e. being without acombined overt level), but consists of e.g. (electro-) magneticallystored information, micro-text invisible to the eye, etc. For bothoptical and other covert layers it is usually possible to devise aprocedure and associated device which performs a fully automaticauthentication.

A more complicated level of security is the forensic level of security.This level is usually only known in detail by a selected group of users,who typically have access to expensive readout equipment. Possibilitiesfor this level are to add tracer molecules to the printing ink, whichcan traced using highly advanced analysis techniques such as NMR scans.Other options are to analyze the exact ink content, or to investigate aspecific size and shape of printed dots.

Finally the biometric level of security allows a feature to store uniqueinformation, typically employed for e.g. tracking and tracing orauthenticating persons. Due to the flexibility of the printing processesof LCP's, it is possible to print e.g. barcodes, serial numbers, imagesand other information.

The availability of additives in the security device can also be used asan additional biometric level. If the additives are not evenly dispersedthroughout the security device, but in a predetermined pattern, they cancontain more information, which can be hidden until a proper read-outmechanism is used. This can be achieved by using different inks tocreate one device, which are in composition equal, except for theaddition of e.g. magnetic, conductive or optical additives.

An example is the local use of magnetic additives in a barcode printedwith nematic LCP inks on a planar aligning reflecting substrate. Thebarcode is printed with two LCP inks which are identical except that onecontains magnetic additives, where the upper half is printed with theink containing magnetic additives and the lower half is printed with theink without the magnetic additives. The barcode can now be inspectedvisually using a polarizer, since the entire barcode is birefringeng.The barcode will be non visible if the polarizer has its polarizationdirection parallel or orthogonal to the alignment direction of the LCP,the barcode turn dark at a 45 degree angle to the alignment direction ofthe LCP. By using a magnetic sensor, such as a magnetoresistive sensor,the upper half can be inspected magnetically, whereas the lower halfwill give no magnetic signal.

1. A security device comprising the following components: (a) 30-80 wt %mono-functional LCP's (b) 0-50 wt % higher functional LCP's, preferablybelow 50 wt % (c) 0-30 wt % liquid crystalline inert monomers,preferably below 20 wt % (d) 0-50 wt % non-liquid crystalline (mono- orhigher) functionalized monomers, preferably below 30 wt % (e) 0-30 wt %non-liquid crystalline inert monomers, preferably below 20 wt % (f)0.01-10 wt % initiators, preferably below 2 wt % (g) 0-10 wt %inhibitors, preferably below 2 wt % (h) 0.01-50 wt % additives,preferably below 20%, more preferably below 10 wt %, with the provisionsthat the total amount of the components is 100 wt %, characterized inthat the additives h) comprise magnetic additives, such as paramagnetic,super-paramagnetic, diamagnetic or ferri-magnetic particles.
 2. Thesecurity device according claim 1 characterized in that the additives h)comprise additionally or only conductive or semi-conductive additives.3. The security device according to claim 2, characterized in that theconductive or semi-conductive additives are selected from a groupcomprising nanometer or micrometer sized rods, flakes, spheres orotherwise suitably shaped conductive particles of metals, alloys orsemiconductor-based materials.
 4. The security device according to claim3 characterized in that the conductive or semi-conductive additives areselected from a group comprising semi-conductive conjugated polymers,such as polyphenylene vinylene semi-conductive liquid crystals, such asoligothiophenes, which are preferably LCP's.
 5. The security deviceaccording to claim 1 characterized in that the additives h) compriseadditionally or only photochromic pigments or dyes, thermochromicpigments or dyes, electrochromic pigments or dyes, ionochromic pigmentsor dyes, halochromic pigments or dyes, solvatochromic pigments or dyes,trobochromic pigments or dyes and piezochromic pigments or dyes.
 6. Thesecurity device according to claim 1, characterized in that thecomponents are applied to a substrate by means of printing, such as butnot limited to inkjet printing, flexography, offset printing, screenprinting, micro-contact printing, micro-transfer printing, gravureprinting, rotogravure printing, reel-to-reel printing.
 7. The securitydevice according to claim 1, characterized in that the components arealigned by a substrate layer comprising linearly photo-polymerizablepolymers.
 8. The security device according to claim 1, characterized inthat the component are aligned by multiple types of alignment throughthe combination of multiple aligning substrates.
 9. The security deviceaccording to claim 1, characterized in that the substrates containfurther authentication features, such as holograms, retro-reflectinglayers, interference stack reflectors, fluorescent layers,color-shifting layers or features printed by means of flakes.
 10. Thesecurity device according to claim 1, characterized in that thecomponents from e) through f) are selected such that they do notprohibit the alignment of the liquid crystals from components a) throughd).
 11. The security device according to claim 1, characterized in thatthe components from a), b) and d) are functionalized to form a polymerstructure, preferably due to possessing compatible types of functionalgroups, very preferable by possessing the same type of functional group.12. The security device according to claim 1, characterized in that thecomponents a) through h) are selected such that phase-separation issuppressed at least only before polymerization but preferably alsoduring polymerization.
 13. The security device according to claim 1,characterized in that the device contains information such as e.g. text,numbers, a barcode, or an image.
 14. The security device according toclaim 1, characterized that the additives are in a pattern so that theycontain additional, optionally hidden, information such as e.g., text,numbers, a barcode or an image.